1. Field of the Invention
The present invention relates to a method for fabricating a capacitor in semiconductor device, particularly to a method for fabricating a capacitor having a Metal-Insulator-Metal (hereinafter, referred to as “MIM”) structure.
2. Description of the Prior Art
In general, MIM capacitors are employed to construct analog circuits of Radio Frequency band in manufacturing semiconductor integrated circuits. An MIM capacitor has a Metal-Insulator-Metal structure having upper and lower electrodes between which an insulator is interposed. Also, the upper and lower electrodes of the MIM capacitor are made from materials which have nearly no depletion and a low resistance. Therefore, the MIM capacitor has a higher Quality Factor (Q) than a PIP (Poly-Insulator-Poly) or PIM (Poly-Insulator-Metal) capacitor.
FIGS. 1A to 1G are sectional views to explain a method of fabricating a conventional MIM capacitor.
First, referring to FIG. 1A, in the conventional method for fabricating an MIM capacitor, a first metal film 100, an insulator film 102, and a second metal firm 104 are sequentially formed on a semiconductor substrate.
The first metal film 100 includes a Ti/TiN film 100a, an Al film 10b, and a Ti/TiN film 100c, which are sequentially formed on the semiconductor substrate. The second metal film 104 has a single layer structure of a TiN film formed on the insulator film 102.
The first metal film 100 may have a lamination structure of either a Ti/TiN/Al/Ti/TiN or TiN/Al/Ti/TiN. In contrast, the second metal film 104 may have a single layer structure made from either Ti/TiN, or TiN, or Al, or W, or another specific metal, or their combination.
The Al film 100a has a low resistance, and thus can function as a layer to transfer a substantial electric signal. The Al film 100a may be replaced by a film made from tungsten (W). The Ti in the Ti/TiN film 100c functions as an adhesive film to improve cohesion between layers on and beneath the adhesive film, which are made from different materials. Further, the TiN in the Ti/TiN film 100c functions as an anti-reflective coat layer which absorbs light to reduce the reflection of light when the photo-resist is patterned.
Meanwhile, the insulator film 102 is formed of an oxide having a high dielectric constant. In general, the insulator film 102 is a film made from silicon-oxy-nitride (SiOxNy) and silicon-nitride (Si3N4) or an oxide film formed by PECVD (Plasma-Enhanced Chemical Vapor Deposition).
Meanwhile, it should be noted that, even though not shown, a contact plug is formed under the first metal film 100 in order to electrically connect the first metal film 100 and a semiconductor substrate mounting integrated semiconductor devices thereon with each other, between which an interlayer insulator film, namely, a PMD (Pre Metal Dielectric) or an IMD (Inter Metal Dielectric) film, is interposed.
Referring to FIG. 1B, photo-resist is applied on the second metal film 104 and is then patterned to form a photo-resist pattern 106 defining an upper electrode of the MIM capacitor.
Referring to FIG. 1C, the upper electrode 104a of the MIM capacitor is formed by dry-etching the second metal 104 by means of plasma activated by combination of Cl2/BCL3/N2 gas while using the photo-resist 106 formed on the second metal film 104 as a mask.
Next, the photo-resist pattern 106 is removed, and then the insulator film 102 is dry-etched using the upper electrode 104a as a mask by plasma activated by combination of CxFy gas which mainly consists of C and F, for example: CF4, C2F6, C4F8, C5F8, etc., so as to form a remaining insulator film 102a. In this case, the remaining insulator film 102a has an undercut “A” formed under the second metal film 104 as shown in FIG. 1c. 
Such an undercut is generated because the insulator film 102 under the second metal film 104 is etched by ion sputtering generated when plasma ions contact with the first metal film 100 during plasma etching.
Referring to FIG. 1D, photo-resist is applied on the entire surface of the resultant lamination obtained through the above process and is then patterned to form a photo-resist pattern 108.
Referring to FIG. 1E, the first metal film 100 is etched using the photo-resist pattern 108 as a mask by plasma activated by combination of Cl2/BCL3/N2 gas, so as to form a lower metal wiring film 110a and a lower electrode 10b of the MIM capacitor. As a result, the first metal film 100 is divided into a lower metal wiring region and an MIM capacitor region. Thereafter, the photo resist pattern 108 is removed.
Referring to FIG. 1f, an interlayer insulator film 112 is formed on a resultant lamination obtained through the above process, and chemical mechanical polishing (CMP) is performed to flatten a surface of the interlayer insulator film 112 while adjusting the thickness of the interlayer insulator film 112. Here, if a single film of an oxide film material is used to form the interlayer insulator film 112, the surface topology of the lower metal wiring region and the MIM capacitor region prevents the interlayer insulator film 112 from being completely flattened.
Accordingly, for the complete flatness of the interlayer insulator film 112, the interlayer insulator film 112 is preferably formed by a single BPSG film or a lamination including at least double films of a PE-TEOS film 112b and a flattening film 112a such as SOG, FOX and FSG.
Referring to FIG. 1g, predetermined portions of the interlayer insulator 112 are selectively etched to form via holes through which portions of the lower metal wiring film 110a and the lower and upper electrodes 110b and 104a of the MIM capacitor. Thereafter, conductive material such as tungsten (W) or copper (Cu) is filled in via holes, so as to form contact plugs 114a, 114b and 114c in the via holes.
Subsequently, a metal film having a lamination structure of Ti/TiN/Al/Ti/TiN, like the first and second metal films 100 and 104, is formed on the entire surface of the resultant device obtained through the process described above, and is then patterned to form upper metal wiring films 116a, 116b and 116c. 
However, in the conventional method for making an MIM capacitor, the undercut generated while the upper electrode is formed functions as a source causing leakage of current when the lower and upper electrodes 110b and 104a of the MIM capacitor are electrically connected with the upper metal wiring film 116b and 116c. 
In order to solve the above-mentioned problems, if the etching quantity of the insulator film 102 is reduced by controlling progress of the plasma dry-etching, a bridge is formed between the lower electrode and the upper electrode of the MIM capacitor when the upper metal wiring film is formed.